1. Field of the Invention
The present invention relates to a drive force transmission apparatus.
2. Description of Related Art
With reference to FIG. 9, a prior art drive force transmission apparatus 100 includes an annular permanent magnet 105 and an electromagnetic coil 106 (see, for example, Japanese Unexamined Patent Publication No. S61-206827A, which was also published as Japanese Examined Patent Publication No. H02-2007B). In this drive force transmission apparatus 100, the annular permanent magnet 105 is clamped between a magnetic plate 102 and a magnetic plate 103, which are held by a rotor 101. The annular permanent magnet 105 generates a magnetic flux (indicated with a solid line in FIG. 9), which flows through an armature 104 and the rotor 101. The electromagnetic coil 106 generates a magnetic flux (indicated with a dotted line in FIG. 9) in a forward direction, which is the same as the direction of the flow of the magnetic flux (indicated with the solid line in FIG. 9) generated by the permanent magnet 105, or in an opposite direction, which is opposite from the direction of the flow of the magnetic flux (indicated with the solid line in FIG. 9) generated by the permanent magnet 105.
The drive force transmission apparatus 100, which uses the permanent magnet 105, normally does not require the electric power except the time of shifting between the clutch on-state (clutched state, i.e., coupled state) and the clutch off-state (declutched state, i.e., decoupled state).
That is, in the drive force transmission apparatus 100, when the electromagnetic coil 106 is energized to generate the magnetic flux in the forward direction, which is the same as the direction of the flow of the magnetic flux generated by the permanent magnet 105, the armature 104, which is spaced from the rotor 101 by a predetermined gap G having a predetermined size, is attracted to the rotor 101 against an urging force of a leaf spring 107. Therefore, the rotor 101 and the armature 104 are coupled with each other (in the clutch on-state). Thereafter, even when the energization of the electromagnetic coil 106 is stopped, the coupling between the rotor 101 and the armature 104 is maintained to maintain the clutch on-state due to the magnetic flux of the permanent magnet 105.
Furthermore, when the electromagnetic coil 106 is energized such that the magnetic flux generated from the electromagnetic coil 106 flows in the opposite direction, which is opposite from the direction of the flow of the magnetic flux generated by the permanent magnet 105, the magnetic flux of the permanent magnet 105 is reduced. Thereby, the armature 104 is displaced away from the rotor 101 by the urging force of the leaf spring 107, and thereby the gap G is formed once again between the armature 104 and the rotor 101 (the clutch off-state). Thereafter, even when the energization of the electromagnetic coil 106 is stopped, the armature 104 is not attracted to the rotor 101 by the magnetic flux of the permanent magnet 105 alone due to the presence of the gap G, and thereby the clutch off-state is maintained.
In some cases, a vibration or an external force may be applied to the drive force transmission apparatus 100, so that the predetermined size of the gap G may not be maintained and may be reduced.
In such a case, in the prior art drive force transmission apparatus 100, even in the state where the energization of the electromagnetic coil 106 is turned off, the magnetic flux, which is formed by the permanent magnet 105, flows like in the case where the clutch on-state is maintained. Therefore, when the size of the gap G is reduced from the predetermined size due to, for example, the vibration, the armature 104 may possibly be attracted to the rotor 101 by the amount of magnetic flux generated from the permanent magnet 105 alone. That is, in the clutch off-state where the energization of the electromagnetic coil 106 is turned off, the unintentional erroneous operational movement causing shifting from the clutch off-state to the clutch on-state may possibly occur.